Temperature and pressure dependence of lattice parameters of the XCu2Si2 compounds (X=Ce,Eu, Yb,La, Gd,Lu, Ca,Th)

The RECU₂Si₂(RE=Rare Earth) series shows pronounced anomalies of the lattice parameters. In those stable valent compounds with 4f orbital angular momentumL0 thec/a ratio shows an anomalous temperature dependence due to crystal field effects. The compounds with unstable valence (RE=Ce, Eu, Yb) show less or no such effects but on the other hand show anomalies of the volume which are temperature dependent. Using these volume anomalies of the unstable valent systems and the measured bulk moduli of the reference compounds we have extracted the valence as function of temperature.Between 4K and 300K the valences of CeCu₂Si₂ and YbCu₂Si₂ change by only a few percent towards 3, while it changes for EuCu₂Si₂ by ca. 20% towards 2. A critical discussion of these valences will be given.     

Anomalies in the temperature dependence of the critical field in antiferromagnetic single crystal dysprosium

An intensive study of the temperature variation of the critical field, H_c, in antiferromagnetic dysprosium, reveals small, abrupt changes in the value of H_c at particular temperatures between the Néel temperature, T_N, and Curie temperature T_c. These measurements made on two single crystals of different quality, show that anomalies occur at ≈99, 114, 127, 131, 138, 146 and 153 K, and the jump in the critical field is ≈ 10^-2 T on each occasion. The anomalies are attributed to a distortion of the helical antiferromagnetic structure whenever the periodicity of the helix is commensurate with that of the hexagonal crystal lattice. Accompanying changes in the helical turn angle, Δω, are estimated and found to increase as the temperature approaches T_c.     

Specific features of magnetic ordering in the SmFeGe2O7 compound

The results of the experimental investigation of the magnetic properties of the SmFeGe₂O₇ compound have been presented. It has been found that the temperature dependence of the susceptibility exhibits two features that coincide with the anomalies in the temperature dependence of the specific heat and indicate magnetic phase transitions in SmFeGe₂O₇. The external magnetic field induces a magnetic transition, the critical field of which depends on the temperature.     

Low-temperature anomalies in thermal expansion of HTSCs: System Bi2Sr2−x LaxCuO6

The data on thermal expansion at low temperatures have been obtained for HTSC single crystals of the system Bi₂Sr_2?x La_xCuO₆ with different doping levels. Anomalous (negative) thermal expansion is observed in the temperature range from 5 to 20 K. It is shown that the anomaly vanishes in an overdoped sample. An anomalously strong effect of magnetic fields of 2–4 T on the negative thermal expansion domain is observed. The effect of field screening, frozen field, doping level, defects and oxygen vacancies on the region of thermal expansion anomalies is investigated. The origin of the observed anomalies in the properties of the system Bi₂Sr_2?x La_xCuO₆, as well as other HTSC systems in which similar anomalies have been observed, is discussed.     

EPR studies of linewidth anomalies at phase transitions in [N(C2H5)4]2MnCl4

We have studied [N(C₂H₅)₄]₂MnCl₄ crystal by X-band CW EPR spectra in the temperature range 170-300 K. The angular dependences of linewidth ΔH were measured and described in the light of a double-layer system (2D) with exchange interactions. Two temperature anomalies of linewidth ΔH were found at T₁=225 K and T₂=192 K on cooling. Different behaviors of ΔH anomalies recorded for an external magnetic field parallel and perpendicular to the ab crystallographic plane indicate ordering/disordering of MnCl₄ groups in this plane and their displacement along the c-axis which occurs in the temperature of about 225 K.     

Anomalies of attenuation and phase velocity of surface acoustic waves in YBa2Cu3O7‒δ thin films in the vicinity of Tc

We present measurements of the attenuation and phase velocity of surface acoustic waves in thin YBa₂Cu₃O films as a function of temperature, in magnetic fields up to 3.6 T applied parallel to the c-axis of the films. We have observed anomalies in both, the attenuation and the phase velocity in the vicinity of the superconducting critical temperature which do not depend on the magnetic field. Possible origins of these anomalies, observed, to our knowledge, for the first time in YBa₂Cu₃O thin films, are discussed and compared to bulk acoustic wave experiments. We present a kind of feedback technique for surface acoustic waves which improves the sensitivity of this type of measurement. The actual sensitivity limits are mentioned. Received: 7 August 1997 / Revised: 7 November 1997 / Accepted: 17 November 1997     

Phase transitions and metastable states in TlGaSe2

The results of measurements of the dielectric constant of TlGaSe₂ in temperature range of successive phase transitions are presented. An anomaly in the temperature dependence of the real part of dielectric constant in TlGaSe₂ has been observed at about 242?K in addition to anomalies at 115, 108, and also near 65?K as reported in previous publications. The presence of temperature hysteresis effects in temperature interval between 115 and 242?K allowed making a conclusion about possible existence of an incommensurate phase in the mentioned temperature range. A model of succession of the structural phase transitions in TlGaSe₂ has been suggested.     

Comparitive study of structure changes with temperature in univalent and divalent ionic nitrate crystals

The temperature dependence of the d.c. resistivity and dielectric constant of polycrystalline samples of Pb(NO₃)₂, Ba(NO₃)₂ and Sr(NO₃)₂ were found to show anomalies in the high temperature region. In addition, sharp peaks were obtained in both differential thermal analysis (DTA) and thermomechanical analysis (TMA) curves in the same temperature range. The observed anomalies were attributed to orientational disorder of the nitrate group leading to an order-disorder phase transition which is reported here for the first time. A comparison is made between the role of NO₃^?ions in both the divalent and univalent nitrates, taking NaNO₃ as representative. The TMA curve for Sr(NO₃)₂ showed a pronounced peak at 325°C. This peak was related to a sudden increase in the expansion coefficient associated with the rotation of the NO₃^? group leading to a solid state phase transformation. Energy diagrams describing the conduction mechanism and showing a fractionization of energy barriers in the case of divalent nitrates are introduced.     

Effect of γ-irradiation on the heat capacity of an [NH2(CH3)2]2 · CuCl4 crystal in the temperature range 80–300 K

The heat capacity of [NH₂(CH₃)₂]₂ · CuCl₄ crystals prior to and after γ-irradiation with doses of 1, 5, 10, and 50 MR is measured by the calorimetric method in the temperature range 80–300 K. It is found that, as the temperature decreases, the temperature dependence C _p (T) exhibits two anomalies which correspond to phase transitions from the incommensurate to the ferroelectric phase at T _c =281 K and from the ferroelectric to the ferroelastic phase at T ₁=255 K. The nature of the anomalies is typical of a first-order phase transition. In addition, a smeared anomaly in the form of a small increase in the heat capacity of the ferroelectric phase is observed at T≈275 K. It is demonstrated that when the dose of γ-irradiation increases, the anomalies decrease in magnitude and the phase transition temperatures are displaced: T _c increases and T ₁ decreases.     

Elastic anomalies in strontium titanate

The anomalies of the second and third-order elastic constants have been considered for the phase transition of strontium titanate within the framework of Landau’s theory. All the anomalies of the second-order elastic constants have been obtained in a single formula using Kronecker delta functions and relations among them have been established. The real parts ofC*₁₁ andC*₄₄ decrease steeply across the transition temperature and thereafter flatly tend to their asymptotic values in the low temperature phase agreeing qualitatively with experimental observations. We have also derived expressions for the third-order elastic anomalies and discussed the temperature variation of the real part ofC*₁₁₁. We have derived expressions for the attenuation of the longitudinal and transverse waves along certain simple symmetry directions and have shown that there is nearly good agreement with experimental observations.     

Acoustic properties of multiferroic PbFe1/2Ta1/2O3

The longitudinal acoustic wave velocity and attenuation in PbFe_1/2Ta_1/2O₃ ceramics have been measured by pulse-echo technique in the temperature range from 4.2 to 530 K. The anomalies observed in the sound velocity and attenuation behavior versus temperature were correlated with Burns temperature, temperature range of the coexistence of relaxor ferroelectric and antiferromagnetic states, and a suggested second antiferromagnetic phase transition at low temperatures.     

Elastic anomaly in K2ZnCl4 around the normal-incommensurate-commensurate phase transition points

The ultrasonic velocities in K₂ZnCl₄ were measured as functions of temperature. At the incommensurate-commensurate phase transition point the velocity of the c₅₅ mode showed a clear anomaly, whereas anomalies were hardly observable in all other modes. The results are analyzed in terms of a phenomenological theory.     

Elastic properties of cubic relaxor ferroelectrics

Comparative analysis of the elastic properties of PbMg_1/3Nb_2/3O₃ (PMN) and PbMg_1/3Ta_2/3O₃ (PMT) crystals has been performed. The temperature dependences of the elastic moduli C ₁₁, C ₁₂, and C ₄₄ are presented. These curves exhibit anomalies near the Burns temperature for both crystals. Additional anomalies are observed in the vicinity of the suppressed ferroelectric transition (T _C ～ 210 K) in PMN. Nontrivial frequency dispersion of the elastic moduli has been revealed for PMN.     

Inelastic neutron scattering from transverse acoustic modes in K2SnCl6

In cubic K₂SnCl₆ the dispersion curve für q q; [110] of [1$\text{1}$0] polarized acoustic phonons has been measured at three temperatures near the phase transition temperature T_c1. The acoustic branch shows no temperature dependence in the low wave vector region and stiffens slightly near the X-point of the fcc-Brillouin zone as the temperature approaches the phase transition T_c1= 262 K from above. The results support a previously developed model on acoustic anomalies in this compound.     

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La_1.85Sr_0.15CuO₄ are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)     

Hydrostatic pressure effect on commensurate—incommensurate transition in K2SeO4-Elastic anomalies studied by Brillouin scattering

A Brillouin scattering study of the commensurate—incommensurate transition in K₂SeO₄ submitted to a hydrostatic pressure was performed. It was found that an increasing pressure lowers the transition temperature (dT_i/dP_i = ?59 K/GPa). The strong anomalies of velocity and damping of the longitudinal acoustic wave propagating along [0 0 1] are weakly affected when pressure acts. The effect of pressure on these anomalies has been calculated on the basis of the lowest order couplings eQ² and e²Q² between strain e and order parameter Q.     

Acoustic anomalies and relaxation dynamics of relaxor ferroelectric Na1/2Bi1/2TiO3 single crystals studied by using Brillouin spectroscopy

Acoustic anomalies of relaxor ferroelectric Na_1/2Bi_1/2TiO₃ single crystals were investigated over a wide temperature range from −196 °C to 900 °C by using Brillouin spectroscopy. The longitudinal sound velocity, the acoustic absorption coefficient and the elastic constant C₁₁ were determined for the acoustic phonon mode propagating in the [100] direction. Two acoustic anomalies, weaker ones at the cubic-tetragonal phase transition temperature of ~540 °C and more pronounced ones at temperatures near 315 °C near the dielectric maximum temperature, were investigated and discussed in relation with the relevant order parameters coupled to the acoustic waves. The relaxation dynamics in the cubic phase were studied based on the flattening of the mode frequency and the half width, which was observed for the first time, and a modified Arrhenius law.     

Magnetic phases in transition metal chloride intercalation compounds of graphite

Magnetic susceptibility measurements on graphite-FeCl₃ (stage 2), graphite-CoCl₂ (stages 2 and 3), and graphite-NiCl₂ (stages 3 and 5) intercalation compounds are reported, showing anomalies associated with magnetic transitions. The effect of magnetic fields on the transition suggests that the magnetic phases in all these systems are similar and independent of the separation of the planes of magnetic ions. The results for all samples for stage ≥ 2 imply two magnetic phase transitions closely separated in temperature.     

Specific heats of the charge density wave compounds o-TaS and (TaSe ) I

Specific heats of the charge-density-wave compounds o-TaS₃ and (TaSe₄)₂I have been measured over the wide temperature interval 10 K-300 K. Both systems exhibit strong non-Debye behavior. Very weak and broad anomalies are observed at the Peierls transition temperatures. For o-TaS₃, the change in the curvature of the specific heat occurs at temperature of 40 K where glass transition has been deduced from dielectric measurements, and an extended scaling analysis suggests that the glass transition is associated with a dynamical cross over in length scales. We briefly discuss the characteristics and physical origins of the anomalies at both the Peierls and glass transitions. Received 5 April 2002 / Received in final form 28 June 2002 Published online 17 September 2002